Microstructural Assessment of the Guinea Pig Sclera Using Quantitative Acoustic Microscopy

The objective of this initial study is to investigate acoustic microscopy as a tool to elucidate the relationship between microscopic structural properties of the sclera and its macroscopic mechanical properties in healthy and myopic conditions. Excised sclera from both eyes of one guinea pig were scanned with a custom-built scanning acoustic microscope (SAM). Prior to euthanasia and excision, the animal wore a plastic diffuser over the right eye for 7 days, resulting in -9.3 diopters of induced myopia. The left eye served as an untreated control. Upon excision, both eyes were flash-frozen, serially-sectioned at 12-mu m intervals, and scanned using SAM. Maps of bulk modulus (K) and mass density (rho) showed a distinctive layered structure in the sclera consisting of a thin outer layer with K= 2.47 +/- 0.09 GPa and rho= 1.06 +/- 0.02g/cm(3) followed by three layers of successively decreasing K towards the vitreous body. In general, the myopic eye was less stiff at vertical distances between 1 and 4 mm from the optic nerve head. This study suggests that the guinea pig sclera has a layered structure with different values of K and rho; these differences may be related to the specific collagen-network structure of each layer.